A Month With Bash — Part 2: Expansions

A Month With Bash — Part 2: Expansions Continuing from where I left off, the next thing I learned was special parameters in bash: "$*" $# $? $@ $N $- $0 Another important concept I picked up is how bash executes shell scripts. Bash is one of those languages that interprets each line as it goes — but it doesn't stop if a line fails. It continues on unless you explicitly set set -o pipefail (or -e , depending on what you want it to catch). Generally, the procedure looks like this: Tokenizing : splitting the line into tokens, usually split using the IFS value. Brace expansion : a mechanism by which arbitrary strings can be generated. echo file { 1,2,3 } .txt ## output: file1.txt file2.txt file3.txt Bash preserves the order from left to right. Tilde expansion : this is where expansion of special symbols takes place. ~ represents the HOME built-in variable ~+ represents PWD , the current working directory and others DIR = ~/Desktop # this is $HOME/Desktop echo " $DIR " Parameter expansion : introduced with the $ symbol. # ${} — the braces can be omitted for normal variables but not for array-type variables Command substitution : very important — it lets you assign the output of a command to a variable, and use commands inside if and for statements. Done with $(command to execute) . week_name = " $( date +%A ) " # gets the current day of the week echo " $week_name " Generally, $() spawns a new shell instance, so it's advisable to avoid it where possible, for latency reasons. Arithmetic expansion : just from the name, this allows evaluation of arithmetic expressions and substitution of the result. It starts with $(( expression )) . There are some rules — bash doesn't support floating point arithmetic natively, so you'd reach for bc if you need it. I won't go deep into that here since this isn't a full bash tutorial. Here's a simple BMI calculator I wrote while practicing this: #!/usr/bin/env bash # script calculates user's BMI and gives a recommendation set -euo pipefail #